LeBron in Philly certainly isn't the most pristine of fits. Then again, while he's not Durant, it isn't like he's Andre Roberson. I don't know what he shoots on catch and shoot 3s, but I can't imagine the sample is very big. I'd bet on LeBron putting in the extra work to get in the vicinity of 40% as a spot up guy. If he could make it work with D-Wade (though at this point we're talking about a different league), I'm not overly concerned about him being redundant with Embiid, who does a lot of work as a screener and a dribble handoff guy that extends out to the perimeter. It won't be nearly as smooth as it was for the Warriors to integrate Durant into the lineup, but it won't be as messy as it was in Miami or when he returned to Cleveland. LeBron secretly has never really played for a team that could function without him. Were he to join a team that I think is headed to the finals, they would be my pick to win the whole thing (and I say that as somebody who thinks last year's Warriors were the greatest team of all-time).
I'm not a cap expert, so you may have a better handle than me on who they could afford and how. That said, with how little money they have on the books, and how many assets they still have, I have to believe they have enough room to add more pieces around LeBron. Losing Redick would be a big blow, but I think he's probably overqualified as the fourth or fifth guy on a super team. They could probably find a shooter or two on the cheap. My hunch is that they'll move Fultz over the summer and try to target some role players and a real guard - a guy like Kemba is there to be had, and while he wouldn't seem ideal at first, they're eventually going to need a player to carry their bench units and attest for how oversized they can be at times elsewhere.
As you alluded to, though, it all comes down to LeBron. If he's LeBron, I'm not picking against him. If he's a slightly lesser version of LeBron, that could be enough to tilt the scales. I really think the mileage is beginning to catch up to him and I think he'd be better served losing in the first round this year rather than carrying that bum team to get swept by the Warriors. If I'm advising him, I tell him it's in his best interests to be on the KG in Boston plan moving forward - 30 minutes a night or so and easing him out of the permanent alpha dog role.